1. Field of the Invention
The present invention relates generally to an optical information device (hereinafter referred to as “optical disk device”) provided with an optical pickup for recording, reproducing, or erasing information on an optical disk-as an optical information medium, and to a recording/reproducing method for recording, reproducing, or erasing information on the optical disk. Herein, the “recording/reproducing” refers to an apparatus or method capable of carrying out one or both functions for the purposes of the present invention. Besides, the present invention also relates to various systems in which the foregoing optical disk device is utilized.
2. Related Background Art
An optical memory technology in which an optical disk having pit-like patterns is used as a high-density, large-capacity memory medium has been put to practical use while the technology has been applied in an increasingly wide range of fields including digital audio disks, video disks, document file disks, and data files. Functions required for carrying out the recording and reproduction of information to and from an optical disk with high reliability are classified roughly into a light collecting function for forming a microspot at a diffraction limit, focus control (focus servo) and tracking control functions of an optical system, and a pit signal (information signal) detecting function.
Recently, a technique of increasing a numerical aperture (NA) of an objective lens that forms a microspot at a diffraction limit on an optical disk by converging an optical beam has been examined with a view to obtaining an even higher recording density of an optical disk. However, a spherical aberration stemming from an error of a thickness of a substrate that protects a recording layer of the optical disk is proportional to the NA raised to the fourth power. Therefore, in the case where the NA is set to be great, for instance, 0.8 or 0.85, it is indispensable to provide a means for correcting a spherical aberration in an optical system. An example of the same is shown in FIG. 14.
In a pickup 11 shown in FIG. 14, 1 denotes a laser light source as a radiation source. A light beam (laser beam) emitted from the laser light source 1 is converted into parallel light by a collimator lens 3, passes through a liquid crystal aberration correcting element (aberration correcting optical system) 4, enters the objective lens 5, and is converged and directed to an optical disk 6. The light beam reflected by the optical disk 6 goes backwards along the foregoing optical path, and is converged by the collimator lens 3. Then, the light is guided by a light dividing means such as a diffracting element 2 toward photodetectors 9 and 10, and is incident thereto. By calculating electric outputs according to respective quantities of light incident on the photodetectors 9 and 10, servo signals (focus error signal and tracking error signal) and an information signal can be obtained. Here, the NA of the objective lens 5 is at least 0.8.
Though not shown, the objective lens 5 is provided with a driving means such as a coil and a magnet, for the focus control for controlling a position of the objective lens in the optical axis direction and the tracking control for controlling a position of the objective lens in a direction perpendicular to the optical axis direction. Besides, though not shown in the figures, either, a transparent substrate is provided over an objective-lens-5-side surface of an information recording layer of the optical disk 6, so that information is protected. Since errors in the thickness and the refractive index of the transparent substrate lead to spherical aberrations, the liquid crystal aberration correcting element 4 corrects a wavefront of the light beam so that reproduction signals are obtained in the optimal state. On the liquid crystal aberration element 4, patterns of transparent electrodes such as ITO are formed, and by applying voltages to the transparent electrodes, the in-plane refractive index distribution of the liquid crystal aberration correcting element 4 is controlled so that the wavefront of the light beam is modulated.
An optical disk device 116 in which such an optical pickup 11 as above is used is shown in FIG. 15. In FIG. 15, 8 denotes an aberration correcting element driving circuit for applying a voltage to the liquid crystal aberration correcting element 4, 117 denotes a motor for rotating an optical disk 6, and 118 denotes a control circuit for receiving signals obtained from the optical pickup 11 and controlling and driving the motor 117, the objective lens 5, the aberration correcting element driving circuit 8, and the laser light source 1. The control circuit 118 causes the laser light source 1 to emit light, drives the motor 117 so as to rotate the optical disk 6, and controls the objective lens 5 according to signals obtained from the optical pickup 11. Furthermore, the control circuit 118 drives the aberration correcting element driving circuit 8 so as to improve information signals obtained from the optical pickup 11.
An optical system used as the optical pickup 11 in the optical disk device 116 is not limited to the optical system shown in FIG. 14, but may be an optical system disclosed by JP 2000-131603A, which is shown in FIG. 16.
In FIG. 16, a laser light source, a collimator lens, and photodetectors of the optical system as the optical pickup are omitted. These may be configured in the same manner as in the optical system shown in FIG. 14. A light beam converted into a parallel light by a collimator lens, not shown, passes through an aberration correcting lens group 201 composed of a negative lens group 21 and a positive lens group 22, and is converged and directed to the optical disk 6 by an objective lens group 202 composed of a pair of a first objective lens 23 and a second objective lens 24. By changing a distance between the negative lens group 21 and the positive lens group 22 of the aberration correcting lens group 201, the spherical aberration of the optical system as a whole is corrected. To change the distance between the negative lens group 21 and the positive lens group 22, for instance, the lens groups may be provided with a driving means 25 and a driving means 26 for moving the same, respectively. Each of the driving means 25 and 26 may be formed with a voice coil, a piezoelectric element, an ultrasonic motor, or a screw feeder.
In the foregoing configuration, normally, the spherical aberration correction is carried out so as to improve the quality of the information signals on the premise that the focus control stably functions on a single information recording surface of the optical disk 6.
However, according to the DVD standard with an objective lens having an NA of 0.6, a two-layer disk having two information recording surfaces also is adaptable. Therefore, with an NA set to be greater, likewise the two-layer disk structure is effective so as to further increase the memory capacity per one optical disk. A two-layer disk 61 is composed of a substrate 62, an L0 layer (first recording layer) 63, an intermediate layer 65, an L1 layer (second recording layer) 64, and a protective layer 66 on a reverse side, which are laminated in the stated order from the optical pickup 60 side, as shown in FIG. 17. The substrate 62 and the intermediate layer 65 are made of a transparent medium such as a resin. Since the intermediate layer 65 is provided between the L0 layer 63 and the L1 layer 64, a thickness from the-optical-pickup-60-side surface of the optical disk 61 to the L1 layer 64 is greater than a thickness therefrom to the L0 layer 63 by the thickness of the intermediate layer 65. This thickness difference generates a spherical aberration. In the case of an optical system according to the DVD standard in which the objective lens has an NA of 0.6, however, the foregoing spherical aberration is within a tolerance, thereby making it possible to record/reproduce information without aberration correction.
In the case where an objective lens with a great NA of not less than 0.8 is used so as to further increase the recording density, a spherical aberration due to the thickness of the intermediate layer 65 cannot be ignored. In other words, it is impossible to record/reproduce information with respect to both recording layers with a common optical pickup without correcting a spherical aberration. In the case where the NA is increased to not less than 0.8, as described above, a spherical aberration correcting means is provided even in the case where information recording/reproduction is carried out with respect to a single recording layer. Therefore, in the case where the recording/reproduction is carried out with respect to the two-layer disk as shown in FIG. 17, the spherical aberration due to the thickness of the intermediate layer 65 is cancelled by optimally carrying out the spherical aberration correction with respect to each recording layer.
To the two-layer disk as shown in FIG. 17, a position at which a light beam is converged thereby forming a microspot (hereinafter referred to as focus position) occasionally is moved: for instance, from the L0 layer 63 to the L1 layer 64 while information is being recorded/reproduced to/from the L0 layer with the light beam converged onto the L0 layer 63, so that information is recorded/reproduced to/from the L1 layer 64; or to the contrary, from the L1 layer 64 to the L0 layer 63. (Such an operation of moving the focus position to another recording layer is hereinafter referred to as “interlayer jump”.) JP 9(1997)-115146A, JP10(1998)-143873A, JP11(1999)-191222A and JP11(1999)-316954A disclose techniques of devising a pulse or an offset signal to be applied to a focus error signal, so as to stabilize the focus control upon such an interlayer jump.
The foregoing documents, however, do not disclose an idea that a correction quantity of the spherical aberration is changed for each recording layer upon an interlayer jump. In the case where the NA is not less than 0.8, when an interlayer jump is made without changing the spherical aberration correction quantity, the following problems arise.
FIG. 18 is a flowchart illustrating an operation when an interlayer jump is carried out. When the control circuit issues an interlayer jump command (or the control circuit receives an interlayer jump command from another circuit) while a recording/reproducing operation is carried out with the focus control being conducted with respect to a first recording layer (hereinafter referred to as “first layer”) (Step 901), the control circuit generates an interlayer jump signal (Step 902), the focus position is moved to the second recording layer (hereinafter referred to as “second layer”) (Step 903), and a recording/reproducing operation is carried out to a second layer (Step 904). FIG. 19 is a timing chart of respective signals in the foregoing operation. The interlayer jump signal varies in response to a signal corresponding to the interlayer jump command at Step 901 as a trigger (Step 902). The interlayer jump signal is, as shown in the figure, composed of a kick pulse KP for leaving a loop for focus control with respect to the first layer and starting to move the objective lens so that the focus position is moved to the second layer, and a brake pulse BP for stopping the moving of the objective lens and entering a loop for focus control with respect to the second layer.
In such an interlayer jump operation, during the recording/reproduction with respect to the first layer before a jump, the spherical aberration correction quantity is optimal with respect to the first layer. Therefore, if the focus position is moved to the second layer without any change to the correction state of the spherical aberration, a spherical aberration occurs due to the thickness of the intermediate layer 65 between the first and second layers. This results in the deterioration of the focus control signal (the deterioration of the amplitude and linearity of the focus error (FE) signal, the occurrence of an offset, etc.), thereby making the focus control with respect to the second layer unstable. Further, though it is effective to refer to a magnitude of a reproduction signal so as to confirm whether or not the focus control functions normally, this also raises the following problem. Namely, if a spherical aberration occurs when the focus position is moved to the second layer, the reproduction signal has a smaller amplitude, thereby making it impossible to check whether or not the focus control is carried out normally.